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Defect Detection

Boeing patent© Kilburn & Strode LLP

Inspection techniques and proof testing methods are often used to identify the presence of inherent flaws and manufacturing defects in components and parts, both before and during service. Internal defects that remain undetected may act as stress intensifiers under applied loads, exhibiting the potential to impair performance of, or cause catastrophic failure to, a material or assembly.

Non-destructive testing methods are commonly used to reveal the presence of surface and sub-surface defects such as voids or micro-cracks in materials. In some instances, certain limitations of inspection and testing methods, however, can lead to defects being undetected and thereby prevailing in parts approved for inservice applications. For example, the use of ultrasonic testing to determine internal defects in parts with complex shape geometries or non-uniform densities can be difficult, requiring extensive analysis. Similarly, the accuracy of X-ray inspection can be somewhat dependent upon the size, shape and orientation of defects with respect to the detector array.

These difficulties may be overcome by a process patented in the UK, in November 2018, by The Boeing Company, which manufactures commercial jetliners as well as defence, space and security systems.

The patent, GB 2548710, titled Radiographic method and apparatus for detection of cracks, defects, or leak pathways in materials and assemblies, discloses a method of testing for internal defects in materials and assemblies that are comprised of permeable surfaces and complex shape geometries or multiple density constituents. This method, therefore, allows for the determination of internal defects that may not otherwise be easily discernible using conventional techniques.

The image to the left, taken from patent GB 2548710, illustrates the patented system (100) whereby a part is first enclosed and sealed in a chamber (102) and subsequently placed under a partial vacuum, through use of a vacuum source (104) such as a vacuum pump. A radioactive or isotope-labeled liquid that is under pressure is then released from a fluid source (106) into the chamber (102) via a fluid pathway (120), permeating and occupying any internal crevices in the part. The time period required for the liquid to permeate the part is dependent upon several factors, including the intrinsic permeability of the part, the properties of the liquid selected, as well as the concentration gradient of the liquid which is modelled using Fick’s law of diffusion.

Exposing the part to a vacuum environment first before introducing the radioactive or isotope-labeled liquid under pressure enables a relatively lower pressure within any internal defects as compared with that in the chamber (102). This means that upon evacuation of the radioactive or isotope-labeled liquid, from the chamber (102) to a fluid reclamation container (108), some of the liquid may be retained in the part indicating the presence of defects. The presence or absence of any of the radioactive or isotope-labeled liquid entrained in the part is given by an indicator (114) and regions of entrapped liquid are then observable using an X-ray detector (110).

The various components of the system, including the vacuum source (104), fluid source (106), fluid reclamation container (108), detector (110), and indicator (114), are all connectable to a controller (112) which controls the process through wired or wireless means.

The development and improvement of inspection techniques and testing methods is invaluable for ensuring the structural integrity of materials and assemblies that govern the safety and reliability of many critical applications. This patented process allows for the X-ray detection of internal defects in materials and assemblies to be facilitated more easily and to a higher degree of accuracy.

The Boeing Company patented this invention in the USA last year valid for 20 years from date of filing – and obtaining a British patent now secures approximately 18 years of exclusivity to commercialise the same technology in the UK before expiry.

Read the full patent here

This article first appeared in the March 2019 issue of Materials World, the member magazine of the Institute of Materials, Minerals and Mining.